Not only as an electron gun, an electron source apparatus to generate and emit an electron beam is widely used as an electron beam supplying source to enter an electron beam to a variety of apparatuses such as an accelerator (an electron synchrotron, a linear accelerator, and the like). Further, recently, development of a small X-ray source using laser inverse Compton scattering is proceeding as an X-ray source of an X-ray apparatus such as an X-ray bioimaging apparatus and an X-ray absorption imaging apparatus. An X-ray source using laser inverse Compton scattering generates a photon beam (an X-ray) having predetermined energy by causing laser light to collide with an accelerated electron beam (inverse Compton scattering). Currently, a small electron source apparatus capable of generating a high-strength and high-quality electron beam for actualizing a small X-ray source and the like described above.
Conventionally, in development of an electron source apparatus, work and the like have been conducted with generation and acceleration of a multi-bunch electron beam using a thermoelectron gun (an electron generation method using a hot-cathode). Under a situation that quality of an electron beam is not sufficiently improved with the thermoelectron gun method, recently, an epochal electron acceleration method and an acceleration cavity being a so-called Brookhaven National laboratory (BNL) type high-frequency acceleration cavity to effectively accelerate electrons in a short distance have been devised. With this method, electron acceleration is performed with a high-frequency electric field. Here, since electric field strength having about 10 or more times higher than electric field strength due to a conventional direct-current electric field can be obtained, downsizing of an electron gun has been expected.
As illustrated in FIG. 8, the BNL type high-frequency acceleration cavity has a basic structure in which a half cell (0.5 cell) 5 and a full cell (1.0 cell) 6 are connected. The length of the half cell 5 on the electron beam axis direction (right-left direction in the drawing) is set to be 0.6 times of the length of the full cell 6 in the axis direction for suppressing beam diffusion. High-frequency power transmitted within a waveguide is supplied into the full cell 6 firstly through a high-frequency power input coupler port 10 (a coupling hole) arranged at a wall face of the full cell 6, and then, supplied to the half cell 5 through an aperture portion (iris) between both the cells. The half cell 5 includes a laser entering port 9 for entering laser light and a detachable end plate 2B. A photocathode 7 made of metal such as Cu and Mg is arranged at the center of the end plate 2B. The end plate 2B is attached to a half cell body via a vacuum seal (helicon-flex seal) 2S and is removed from a half cell body when replacing the photocathode 7 for maintenance or research on cathode material. Adjustment of a resonance frequency of the half cell 5 is performed by increasing and decreasing a cell volume with adjustment of fastening torque of the helico-flex seal 2S. Meanwhile, adjustment of a resonance frequency of the full cell 6 is performed by adjusting positions of adjusting rods (two in total as being symmetrically arranged) capable of being moved up and down respectively within a tuner hole having a diameter of 10 mm formed at a cavity wall of the full cell 6.